This invention is generally in the field of metal coatings and is in particular a method for creating a metal coating on electrically conductive as well as non-conductive substrates.
There are many instances in which it is desirable to form a metal coating on either an electrically non-conductive (i.e., dielectric) substrate or a metal substrate. A number of methods has been used with a variety of materials for this purpose. One commonly used technique is electroplating. Others include chemical vapor deposition. Deposition technologies are reviewed in detail in Deposition Technologies for Films and Coatings by R. F. Bunshah et al. (Noyes Publications, Park Ridge, N.J. 1982).
Electroplating is a process generally used to coat an object with a thin layer of a metal. The object to be plated is charged as the cathode and is immersed in an electrolyte bath which, among other things, contains a salt of the metal being plated. The anode can be made of the same metal or some chemically unaffected conductor. A low-voltage current is passed through the bath to electrolyze it and plate the cathode with the metal to the desired thickness. A firm bond between the metal being deposited and the article is best obtained when using two metals that tend to alloy, such as silver and copper. Disadvantages to electroplating include stringent substrate requirements, such as the type of metal that can be plated, the necessity for absolute cleanliness, and difficulties in plating uneven surfaces. Other disadvantages include the necessity of uniform temperature control and replenishment of the electrolyte bath.
Chemical vapor deposition is frequently used to produce a very thin metal coating on a substrate, particularly a substrate formed of a non-conductive material such as silicon. In this method, a volatile compound of the metal is vaporized at high temperatures (e.g. 800.degree. to 900.degree. C.) and subsequently decomposed in such a way that the metal itself is deposited onto the substrate. Unfortunately, at these high processing temperatures the metal coating diffuses into the underlying substrate at the same time as the deposit is forming. The result can be a diffuse rather than sharp interface between substrate and coating. Sharp interfaces improve electronic device performance in most applications.
Liquid phase epitaxy (LPE) is another method by which metal coatings can be deposited on a substrate. In this method, a first metal is dissolved into a molten bath of a second metal. A substrate, maintained at a temperature below the melting point of the first metal, is then immersed in the molten metal bath. This causes the first metal to leave solution and deposit upon the substrate. Unfortunately, it is impossible to prevent some of the second, carrier metal from becoming entrained and deposited upon the substrate with the first deposit metal. This result restricts the use of LPE in certain applications in which the specified impurity levels are too low to be met by this method.
In general, many of the methods in use are limited to conductive metal substrates or substrates that can tolerate very high temperatures. Some of these methods use health as well as safety hazards due to the need to work at high temperatures, non-atmospheric pressures and with toxic chemicals.
Thus, a need exists for a method by which pure metal coatings can be deposited upon both conductive and non-conductive substrates to provide a sharp interface. A further need exists for a method in which toxic vapor generation and handling problems are minimized.